The present invention relates to a method for laser light irradiation (so called laser annealing) in fabrication of a semiconductor device, and to a method for fabricating a semiconductor device by laser light irradiation, wherein mass production is high, characteristics are little different among the devices, and a production yield is high. More particularly, the invention relates to a method for improving or recovering (repairing) crystallinity of a semiconductor material by irradiating the material with laser light. The semiconductor material includes a semiconductor material having wholly or partially amorphous components, a substantially intrinsic polycrystalline semiconductor material, and a semiconductor material whose crystallinity has been severely deteriorated by damage due to ion irradiation, ion implantation, or ion doping.
Recently, researches have been conducted concerning low temperature semiconductor device fabrication processes mainly because it has become necessary to form semiconductor devices on an insulating substrate made of glass or the like. Also, miniaturization of devices and making of a multilayer structure have required.
In semiconductor fabrication processes, it may be necessary to crystallize an amorphous component contained in a semiconductor material or an amorphous semiconductor material. Also, it may be necessary to recover (repair) the crystallinity of a semiconductor material deteriorated by ion irradiation. Furthermore, when there exists crystallinity, it may be required to be enhanced. In general, thermal annealing is used for these purposes. When silicon is used as a semiconductor material, it is annealed at 600 to 1100.degree. C. for 0.1 to 48 hours or longer. As a result, the amorphous component is crystallized, the crystallinity is recovered, or the crystallinity is improved.
In thermal annealing, as higher temperature is used, the processing time can be shortened. However, at 500.degree. C. or lower, almost no effect produces. At about 600.degree. C., a long processing time is needed. Accordingly, it has been required that the thermal annealing be replaced by other means in order to lower the process temperature. Hence, a laser irradiation technique has attracted attention as an ultimate low temperature process. Since laser light can be irradiated only onto a region that needs high energy corresponding to the energy of thermal annealing, it is not necessary that the whole substrate be processed to a high temperature.
Generally, two methods have been proposed to irradiate laser light. First method uses a continuous oscillating laser such as an argon ion laser. The laser beam having a spot shape is irradiated to a semiconductor material. In this method, the semiconductor material is melted by variations in the energy distribution inside the beam and by movement of the beam, and then the material is solidified mildly. As a result, the semiconductor material is crystallized. Second method employs a pulse oscillating laser such as an excimer laser. The pulse laser having high energy is irradiated to a semiconductor material. In this method, the material is momentarily melted and solidified, whereby the material is crystallized.
The first method has the problem that it takes a long time to perform the processing, for the following reason. Since the maximum energy of a continuous oscillating laser is limited, the maximum beam spot size is several millimeters. On the other hand, in the second method, a large spot of several cm.sup.2 or more can be used because the maximum energy is very large. Hence, the productivity can be improved.
However, in order to process one large area substrate with a normally used beam having a square or rectangle form, it is necessary to scan (move) the beam vertically and horizontally. This produces limitations on the productivity. This problem can be solved by modifying the cross section of the beam into a linear form, making the width of the beam larger than the size of the substrate to be processed, and scanning this beam.
The remaining problem for improvement is uniformity of the effect of laser irradiation. A pulse laser somewhat varies in energy from pulse to pulse and so it is difficult to uniformly process the whole substrate. Especially, it is important to obtain uniform the characteristics of regions where adjacent laser spots overlap each other.
Also, when a pulse laser is irradiated, even if the uniformity of the energy inside the beam of one shot pulse can be accomplished by improvements in the optical system, it is difficult to reduce variations in the characteristics of devices due to overlap of pulse laser. Especially, where devices are located just at ends of the beam of laser light, the characteristics (especially the threshold voltages of MOS transistors) vary considerably from device to device.
In semiconductor devices, considerable variations in the threshold voltages of digital circuits are admitted. In analog circuits, the difference between the threshold voltages of adjacent transistors is required to be 0.02 V or less.
It has been reported that if weak pulse laser light is preliminarily irradiated before irradiation of strong pulse laser light, the nonuniformity is lowered and the uniformity is improved. However, overlap of laser spots has not been discussed sufficiently.